The invention relates to a method of fabricating and to the set-up of an active matrix display device formed of a plurality of pixels and comprising at least one thin film transistor element (in the following TFT element) on a first substrate for each pixel, a low work function material layer, in particular a structured cathode layer forming or contacting a pixel electrode layer, an active organic and/or polymeric electroluminescent material layer (EL layer) covering at least said low work function conducting layer, an electrically conducting high work function layer, in particular an anode layer on said EL layer and structured into elements desired for display as well as a second substrate covering said layered arrangement.
Light-emitting devices based on organic and polymeric electroluminescent (EL) materials are known (see Lit. [1]). For achieving a specific colour emission efficiency of such types of light-emitting devices, WO 96/03015 A1 describes an advanced fabrication process for a conjugated polymers based two part integrally connected light-emitting device, in which each polymer layer is separately pretreated, e.g. by a stretching process. The advantages of a polymer based light-emitting devices, in particular diodes, are high brightness with low power consumption and low driving voltages. The device structure is a relatively simple metal-polymer-transparent electrode sandwich wherein the material of the transparent electrode may be indium tin oxide (ITO). It is there-fore realistic and principally known to combine light-emitting polymer EL devices with active matrix driving like thin film transistors (TFTs) as proposed for example in EP-0 845 770 A1 or U.S. Pat. No. 5,747,928.
Typically, such devices require two electrodes of differing work function, at least one of which is transparent: one high work function anode (e.g. ITO, fluorine-dopen tin oxide, or gold) for hole injection and a low work function cathode (e.g. Mg, Al, Li, Ba, Ca) for electron injection into the organic or polymeric material. Up to now the transparent electrode in efficient devices is always the anode, which is in most cases applied to the substrate before diode preparation. Sputtering of ITO onto a finished device has been used, but the efficiency is poor and such processes are expected to damage the active polymer or organic layer. Frequently, but not necessarily, such devices comprise separate layers for electron and hole injection transport as for example proposed in the above mentioned WO-document, and occasionally in addition they also sometimes comprise an additional light-emitting layer sandwiched between the hole- and the electron-transport layers (see Lit. [2]). Flexible devices on polymeric substrates have also been reported wherein such substrates are coated with a high work function electrode, usually ITO and/or polyaniline (see Lit. [3]).
Active matrix liquid crystal displays (LCDs) driven by TFTs are commercially widespread, for example in notebook computers. In TFT/LCDs, an abbreviation of thin film transistor-addressed liquid crystal displays, each pixel element (pixel) is controlled by a thin film transistor. TFT/LCDs create a whole new world of technology in consumer electronics and in computer and communication systems. The market for TFT/LCDs is now growing much faster than expected and has an impact on new application fields, as well as conventional fields.
The structure of a single TFT in a matrix type arrangement of hundreds of thousands of TFTs is a FET (field effect transistor) and a pixel electrode. The pixel electrode is contacted to the source (or drain) electrode of the FET, and thus the effective window area (aperture ratio) is reduced by the size of the transistor area. The aperture ratio governs the brightness of the panel, thus the larger the aperture ratio becomes the brighter the display panel is achieved.
The concept of TFT/LCDs is not new, but rather old. As early as 1966 Weimer proposed the possibility of using TFTs as display switches (see Lit. [4]). A more detailed concept was described in 1971 (Lit. [5]), where the use of diodes or triodes (transistors) was discussed as switches for active matrix liquid crystal displays. The use of storage capacitors inplemented in parallel with the liquid crystal cell capacitor was also mentioned.
Hydronated amorphous silicon (a-Si:H) was a late arrival in TFT technologies. However, it had a great influence in achieving practical TFT/LCDs. Since the first report by the Dundee group (Lit. [6]), a-Si:H-TFT has been recognized as a suitable device for TFT/LCDs. So far the combination of TFT and LCD technologies has been greatly growing and the market is already rather large.
However, the principle problems of TFT/LCDs are
a large viewing angle dependence if the LCD due to the application and use of twisted nematic (TN) type liquid crystals,
considerable dependence of the switching speed on temperature because the switching is greatly dependent on the viscosity of the liquid crystal itself, and
the liquid crystal injection process, necessary for cell filling takes several hours.
As mentioned above, the demand for portable uses of flat panel displays is increasing leading to the request for thinner and lighter flat panel displays. One approach to this goal are the polysilicon-based TFT technologies which are also progressing, especially with a proposal for integrating the required shift register within the TFT panel, thus reducing the number of connection lines of the TFT panel.
Recently a poly-Si-TFT-addressed polymeric EL display was reported by Cambridge Display Technology. Also known are active matrix driven displays based on polymeric EL materials, wherein the active driving elements are thin film field effect transistors (TFTs) of polysilicon or organic TFTs based on oligothiophene (Lit. [5]). In these reports the TFIs are deposited onto the transparent substrate before preparation of the EL devices.
Also known are techniques to modify the work function of metallic and semiconductor surfaces by attachment of functionalized dipolar layers, e.g. through chemisorption or electrochemical attachment (see Lit. [6]). Such modification has been shown for materials such as ITO, CdTe and CdS. An LED device comprising a modified ITO electrode was recently reported (see Lit. [7]). According to theoretical analysis, the change in the work function is proportional to the dipole moment of the attached molecules and their concentration, and is inversely proportional to their dielectric constants.
Lamination is a well-known technique for combining desired qualities of two or more different materials into a composite layer system and involves joining of the layers under application of pressure and/or heat (see Lit. [8]). Preparation of photovoltaic cell based on polymers by laminating two parts together was recently reported by Friend et al. (see Lit. [9]). Cabrera et al. (see Lit. [10]) reported a class of biftnctional materials for non-linear optics which comprised an aromatic group, particularly a styryl group, functionalized at the 4 and 4xe2x80x2 positions by trifluorosulfonate groups as an electron acceptor at one end and electron donor groups such as ethers, thioethers and amines on the other end. These materials exhibit high dipole moments while showing a relatively small visible light absorption. Due to the possibility of attaching further functional groups, e.g. alkyl chains, to both donor and acceptor groups it was possible to determine the direction of the dipole moment relative to the second function group. No applications of these materials in EL devices have been reported.
Bloor et al. (see Lit. [11]) have reported a class of molecules derived from TCNQ which exhibit dipole moments of up to 25-30 Debye. No applications of these materials to EL devices have been reported.
In addition to the conceptional inconsistencies of light-emitting devices briefly mentioned above, having regard to the problems of which the invention offers a solution, the following disadvantages, shortcomings or needs of the state of the art have been discovered by the inventors.
a. The low work function cathode materials needed for high efficiency devices are typically unstable against oxidation by oxygen or water. Because of this, such devices and displays comprising a matrix of such devices must be thoroughly encapsulated to eliminate water and oxygen from the device. This is particularly difficult when flexible devices on polymeric substrates are desired (see e.g. U.S. Pat. No. 5,747,928), because a transparent, flexible and thoroughly impervious barrier layer must be applied to the side of the device comprising the transparent electrode, and thus far no such materials with sufficient barrier properties have been reported.
b. A further disadvantage of the use of oxidation sensitive cathode materials is there incompatibility with standard photolithographic processing, which will be necessary to achieve high resolution multicolour displays at reasonable prices.
c. Under the current state of the art, it is necessary for the achievement of high efficiency devices to prepare the devices onto a transparent anode material deposited onto a substrate and to evaporate the cathode onto the organic and/or polymeric layers. This eliminates the possibility to modify the cathode work function by chemical means and to use more stable materials.
d. Furthermore, according to the state of the art, in active matrix displays comprising organic and/or polymeric EL devices it is necessary to apply the (non-transparent) TFTs to the substrate before preparation of the EL device. This is a problem because in emissive displays, in which each pixel element consists of a different device element like a TFT, it is difficult to obtain uniformity over the screen, because of the deviation of the each element""s electrical porperties. In order to circumvent a deviation of the brightness over the screen, one should perform some compensation by a built-in circuit in or for each pixel element. If the additional circuits are fabricated in the TFT pixel, it means a reduction of the aperture ratio. With the conventional TFT concept, the aperture ratio is reduced even more with an increase of the resolution or number of the pixels on the panel.
With the above discussed observations and drawbacks in mind, it is an object of the invention to teach a modified or new manufacturing process and a new structure for active matrix display devices offering high resolution displays with a good uniformity of brightness over the screen and simultaneously showing a high aperture ratio.
The invention concerns displays comprising devices which contain organic or polymeric EL materials and its preparation and application, wherein solution to the problems and the objects described above can be achieved.
A method of fabricating an active display device according the invention is defined in claim 1. Advantageous modifications and embodiments of such a manufacturing method are the subject-matter of dependent claims.
An active matrix display device and its specific structure according to the invention is the subject-matter of claim 10 with advantageous modifications and improved embodiments being defined in further dependent claims.
As mentioned above, the combination of TFTs and LEPs (light-emitting polymers) is promissing in terms of eliminating the viewing dependence and temperature dependence of the response speed in TFT/LCDs, which are crucial problems for displays available on the market. However, the current state of the art leads to poor aperture ratios for high resolution displays based on polymeric EL material.
The invention as defined in at least one of the appended claims solves the above problems by metal (pixel electrode) fabrication on the TFT side of one of the substrates, such that at least a major part of each TFT is burried under a pixel electrode allowing for an active matrix addressing of the polymeric EL pixels while maintaining a high aperture ratio even at high resolution.